A wide variety of techniques and process have been developed in the prior art for forming a wide variety of printed circuit boards having various design characteristics for different applications.
These printed circuit boards are formed from layers of different materials which are laminated together under relatively high pressure and temperature in order to form the printed circuit boards. The printed circuit boards or laminates typically include a core material or dielectric formed, for example, from epoxy impregnated glass cloth layers which are laminated together with other circuit forming components, conductors, etc. in the finished printed circuit boards.
Although the present invention is directed toward a method for fabricating such printed circuit boards, the actual construction features of the printed circuit boards themselves are not a feature of the invention. Generally for purposes of the present invention, it is sufficient to understand that the printed circuit boards are formed from laminated layers as discussed above, a substantial portion of those layers involving core material formed for example from glass cloth and epoxy resin as discussed above. As will be noted in greater detail below, the epoxy resin is particularly troublesome during formation of the printed circuit boards since it tends to be migratory and frequently contaminates molding surfaces in contact with the printed circuit boards or laminates.
Typical methods for forming printed circuit boards are discussed for example in U.S. Pat. No. 4,201,616 issued May 6, 1980 to Chellis, et al. and U.S. Pat. No. 4,753,847 issued June 28, 1988 to Wilheim, et al. as well as a substantial number of other U.S. patents cited or discussed within the above two references. Both of the above references as well as the references cited or discussed therein are incorporated by reference in this application as though set forth in their entirety in order provide a more complete discussion of background material in connection with the present invention.
In molding printed circuit boards or laminates as discussed above, the various layers or laminates of the respective printed circuit boards are arranged together and pressed under relatively high pressure and temperature as noted above in order to produce the finished board. Futhermore, for purposes of economy, multiple combinations of laminates are commonly formed into respective printed circuit boards simultaneously between a pair of mold dies or the like. In such operations, separator mold plates, also sometimes referred to as caul plates, planishing plates, etc. are employed between the respective printed circuit board or laminate assemblies as well as between the mold dies and surfaces of adjacent printed circuit board laminates.
Within such molding operations, it is also common practice to employ tooling pins extending in spaced apart relation between the molding dies and through both the laminate assemblies and the separator plates in order to assure that all of the components remain in proper alignment during the molding operation. The holes provided in the separator mold plates for accommodating these tooling pins present particular problems as noted in greater detail below.
Generally, it is desirable to re-use the separator mold plates which are commonly formed from hard metals such as stainless steel, spring steel, aluminum, etc., again for purposes of economy. At the same time, it is necessary to assure that the surfaces of the separator mold plates are clean so that they form generally true and accurate surfaces on the laminate structures. At the same time, it is also necessary to assure that foreign material is not transferred to the laminates from the separator mold plates during the molding operation.
For this reason, mold release agents have been occasionally employed on the surfaces of separator mold plates or between separator mold plates and the laminate assemblies to prevent such undersirable transfer of contaminating materials. However, it is also to be noted that the mold release agents themselves may be contaminating materials inasfar as the laminate assemblies are concerned. Accordingly, it is also important to assure that the mold release agent is not deposited upon or transferred to the surface of the printed circuit board laminate formed during the molding operation.
The Wilheim, et. al. patent referred to above is typical of prior art processes wherein separate sheets of mold release material are arranged between the separator mold plates and the laminate structures. In particular, that patent discloses the use of separate mold release sheets formed from a film of polyester, nylon or cellulose acetate treated for promoting resin adhesion and having a coating of a thin release layer of a cured release acrylated oligomer resin on at least one side of the film.
Although such mold release sheets or film are effective when properly employed, they tend to be expensive in molding operations such as those discussed above because of labor intensive steps of properly arranging the mold release sheets in place prior to each molding operation. At the same time, the separate sheets commonly result in surface defects in the printed circuit boards or laminates usually due to irregularities resulting from laying down of the mold release sheets or films. For example, manual laying down of the film commonly results in creases or wrinkles, overlapping seams, discontinuities and other surface irregularities which also tend to appear in the surfaces of the printed circuit boards or laminates formed by the process.
Furthermore, since the film is not integrally associated with the separator mold plate, discontinuities or defects in the sheets may permit materials such as the migratory epoxy referred to above to penetrate onto the surface of the separator mold plate. Since the epoxy material interferes with the necessary smooth surface of the plate and also because the epoxy tends to strongly adhere to the metal plates, it is therefore commonly necessary to remove the epoxy by abrasive grinding between molding operations. The abrasive grinding is not only time consuming and expensive in connection with the molding operation, it also tends to abrade the surface of the mold release plates and to reduce their normal operating lifetime in such molding operations.
Other prior art techniques have comtemplated the use of a wide variety of mold release agents integrally applied to the surfaces of the separator mold plates. U.S. Pat. No. 4,371,476 issued Feb. 1, 1983 to Newkirk, et. al. disclosed a variety of mold release agents contemplated for applications other than the molding of printed circuit board laminates as contemplated by the present invention.
Prior art mold release agents applied directly to the surfaces of the separator mold plates were generally applied either as part of sequential molding operations or permanently applied, for example, as flame-sprayed powder coats. Generally, the prior art mold releases applied to the separator mold plates as part of the molding operations have tended to suffer from discontinuities, surface irregularities and poor mold release characteristics. Such surface irregularities may include puddling of the mold release agent and unacceptable variations in thickness, both of these irregularities tending to arise during applications of the mold release agent to the plates. These mold release agents applied to the plates as part of the molding operation have also tended to undesirably transfer to the printed circuit board laminate surfaces and to detract from their subsequent processability.
Where the mold release agents have been permanently applied, for example, as flame-sprayed powder coats, as precoatings, they have tended to be relatively thick and also to suffer from development from surface irregularities, for example due to chipping of the mold release agent. Failure of the precoated mold release agents was particularly expensive and time consuming since it generally involved the need for completely stripping the precoated mold release agent from all surfaces of the separator mold plate and then replacing and curing a coating of the mold release agent before returning the separator mold plates to the molding operation.
Repeated application of such materials to the plates tended to produce surface irregularities upon the plates, often because of substantial build-up, and also tends to result in undesirable transfer of contaminating materials to the printed circuit boards.
In other cases, mold release agents were applied as permanent precoats (flame sprayed powder coatings) to the separator mold plates and anticipated to last through large numbers of repeated mold operations. However, these precoated separator mold plates have been found to be relatively expensive and subject to the development of discontinuities in the mold release material due, for example, to chipping, etc. Thus, it was again relatively expensive and time consuming to maintain the separator mold plates in condition for large numbers of sequential molding operations as referred to above.